The polyphenylene ethers are known and described in numerous publications, including Hay, U.S. Pat. Nos. 3,306,874 and 3,306,875; and Stamatoff, U.S. Pat. Nos. 3,257,357 and 3,257,358, all incorporated herein by reference. They are useful for many commercial applications requiring high temperature resistance and, because they are thermoplastic, they can be formed into films, fibers and molded articles. In spite of these desirable properties, parts molded from polyphenylene ethers are somewhat brittle due to poor impact strength. In addition, the relatively high melt viscosities and softening points are considered a disadvantage for many uses. Films and fibers can be formed from polyphenylene ethers on a commercial scale using solution techniques, but melt processing is commercially unattractive because of the required high temperatures needed to soften the polymer and the problems associated therewith such as instability and discoloration. Such techniques also require specially designed process equipment to operate at elevated temperatures. Molded articles can be formed by melt processing techniques, but, again, the high temperatures required are undesirable.
It is known in the art that the properties of the polyphenylene ethers can be materially altered by forming compositions with other polymers. For example, Finholt, U.S. Pat. No. 3,379,792, discloses that flow properties of polyphenylene ethers are improved by preparing a composition thereof with from about 0.1 to 25 parts by weight of a polyamide. In Gowan, U.S. Pat. No. 3,361,851, polyphenylene ethers are formed into compositions with polyolefins to improve impact strength and resistance to aggressive solvents. In Cizek, U.S. Pat. No. 3,383,435, incorporated herein by reference, Fox, U.S. Pat. No. 3,356,761, and Bostick et al, French Pat. No. 1,586,729, there are provided means to simultaneously improve the melt processability of the polyphenylene ethers and upgrade many properties of polystyrene reins. These patents disclose that polyphenylene ethers and vinyl materials, e.g., blended or grafted polystyrene resins, including many modified polystyrenes, are combinable in all proportions to provide compositions having many properties improved over those of either of the components. This invention provides compositions of the type disclosed broadly in such prior art, but with unexpectedly high impact strength.
Preferred embodiments of the Cizek patent are compositions comprising a rubber modified high-impact polystyrene and a poly(2,6-dialkyl-1,4-phenylene)ether. Such compositions are important commercially because they provide both an improvement in the melt processability of the polyphenylene ether and an improvement in the impact resistance of parts molded from the compositions. Furthermore, such compositions of the polyphenylene ether and the rubber modified high-impact polystyrene may be custom formulated to provide pre-determined properties ranging between those of the polystyrene resin and those of the polyphenylene ether by controlling the ratio of the two polymers. The reason for this is that the Cizek compositions exhibit a single set of thermodynamic properties rather than the two distinct sets of properties, i.e., one for each of the components of the compositions, as is typical with compositions or blends of the prior art.
The preferred embodiment of the Cizek patent is disclosed to comprise poly(2,6-dimethyl-1,4-phenylene) ether and a rubber modified high-impact polystyrene (identified in Example 7 as Lustrex HT88-1 of Monsanto Chemical Company). It is known in the art that Monsanto HT-88 high impact polystyrene contains an elastomeric gel phase dispersed through a polystyrene matrix and that this elastomeric phase comprises about 12-21% by weight of the composition. A 20.7% gel content is shown, e.g., in Table 3 in Vol. 13, Encyclopedia of Polymer Science and Technology, 1970, p. 401 et seq. Thus the preferred embodiment of the Cizek patent, which was disclosed to have a notched Izod impact strength ranging from 1.05 to 1.5 ft.lbs./in. notch (Standard Method, ASTM-D-256) comprised a polyphenylene ether and a rubber modified high-impact polystyrene resin, the polystyrene containing less than 21% by weight of a dispersed elastomeric gel phase. In addition, it is known that in the gel free polystyrene matrix in Lustrex 88, the weight average molecular weight, M.sub.w, is about 251,000 and the number average molecular weight, M.sub.n, is about 73,000, and, therefore, the polydispersity, i.e., the ratio M.sub.w /M.sub.n is about 3.44.
It is generally recognized that the properties of impact resistant polystyrenes are highly dependent on the number, size and character of dispersed elastomeric particles. Moreover, while most commercial impact polystyrenes contain from 3 to 10% by weight of polybutadiene or rubbery butadiene-styrene copolymer, polystyrene grafted or occluded in the rubber particles gives rise to a gel content, i.e., fraction of the reinforcing rubber (elastomer) phase of 10-40%. Means are known to vary the gel content, while holding the rubber content constant, generally comprising stirring a mixture of styrene monomer and rubber during the early stages of polymerization, rapid stirring giving the lowest gel content and slower stirring giving progressively greater gel content and with no stirring at all, the gel content is &gt;80%. Means are also known to vary the molecular weight distribution, i.e., polydispersity, of the polystyrene matrix. Generally, longer polymerization time at lower catalyst levels and more moderate temperatures provided the highest ratio of M.sub.w /M.sub.n, i.e., the low molecular weight polystyrene fraction is decreased by such techniques. With respect to gel content, it has been the desired objective to maintain this relatively low, and, it is to be noted that the high impact polystyrene used in Cizek's optimum compositions has a gel content of 12 to 20.7% by weight. As to the molecular weight distribution of polystyrene in the matrix, i.e., polydispersity, it has also been the desired objective to keep this narrow to minimize the amount of low molecular weight "tail" fraction which lowers mechanical properties, especially impact strength. The high impact polystyrene used in Cizek's optimum compositions has a polydispersity of below 3.5, i.e., 3.44, see the above citation.
In the present state of the art, therefore, compositions of polyphenylene ethers and rubber modified styrene resins are known, in which the polystyrene resin contains a dispersed elastomeric gel phase of 12 to 20.7% by weight, and which have a notched Izod impact strength of from 1.05 to 1.5 ft.lbs./in. notch. It is also known that in the particular rubber modified polystyrene used, the polydispersity of the gel-free polystyrene matrix, M.sub.w /M.sub.n, is 3.44.
In view of the above, it has now unexpectedly been found that compositions of a polyphenylene ether with a rubber modified polystyrene resin can be provided with substantially improved impact strengths if the dispersed elastomeric gel phase comprises at least 22% by weight of the rubber modified polystyrene. It is preferred that the gel free polystyrene in the matrix have a broad molecular weight distribution, too. The polydispersity (M.sub.w /M.sub.n) should be not less than about 3.5, in contrast to the maximum of 3.44 used in the prior art. The impact strengths of the present compositions are substantially higher than those of comparable compositions wherein the gel content of the rubber modified high impact polystyrene is from 12 to about 21%, i.e., within the range disclosed in the prior art. In addition, the surface appearance, especially gloss, is unexpectedly improved, as is the resistance to aggressive solvents, such as gasoline.